Sound collecting system



Oct. 8, 1940- E. w. KELLOG'G SOUND COLLEQTING SYSTEM Filed Dec. 51, 1937 Patented Get. 1940 to Radio Corporation of America, a.

of Delaware Application December 31, 1937, Serial No. 182,720

6 Claims.

This invention relates to sound collecting systems, and more particularly to a microphone system of the directive type.

Many attempts have been made to provide a microphone which is sensitive to sound coming from a particular source and in a particular direction and insensitive to sounds coming from other directions. Among the proposals heretofore made has been the almost obvious one of in placing the microphone at the focal point of a large parabolic reflector with the reflector facing the direction from which it is desired to pick up the sound. The results have not usually been satisfactory, however, with such a system probably because, if the parabola is made large enough to give appreciable concentration of low frequency sounds, it becomes so sharply directive for high frequency sounds that it can not be aimed sufli ciently accurately. For example, the directive characteristics of such a system show that, at the high frequencies, certain components will be cancelled out if the source is not exactly on the axis. One attempt to lessen this effect at the high frequencies consists in using a parabola of smaller base diameterand of considerable depth or length. However, it has been found that with such a reflector, objectionable resonances occur. When deep reflectors have been used, resort has been had to attaching'Helmoltz resonators near the microphone so that the excess sound will be absorbed at certain frequencies. Another fault which is almost always encountered when sound concentrators of the lens or the parabolic reflector type are used is that the high frequency components are strongly reinforced while those of lower frequency are less affected, with the result that 'the sound picked up by the microphone is badly unbalanced. If the reflector is made large enough to be effective at low frequencies. the concentration of the high frequency components is still further increased.

The primary object of my present invention is to provide an improved sound collecting system of the type under consideration which will be free from the foregoing and other similar defects of prior art systems.

More specifically, it is an object of my present invention to provide an improved sound collecting system as aforesaid which will be sum- 50 ciently directive for both high and .low frequency sounds without having such extreme directivity at certain frequencies as to make it difiicult to secure good results. Another object of my present invention is to corporation which need not be aimed with extreme accuracy toward the sound source in order to receive sound therefrom in the desired direction.

Still another object of my present invention is to provide an improved sound collecting system of the type described which will have a characteristic free from objectionable resonant peaks.

It is also an object of my present invention to provide an improved sound collecting system as aforesaid which is extremely simple in construction and highly efficient in use.

In accordance with one form of my present invention, I provide a parabolic reflector having a. large diameter and place a suitable microphone substantially at the focus thereof. The large diameter is necessary to obtain reinforcement and directivity at low frequencies, but since the use of only a small portion of the reflector is desirable for the high frequency sounds, I line the interior surface of the reflector in a region near its outer edge with a suitable sound absorbing material which will absorb the high frequency sounds much more than the low frequency sounds. For this purpose may be used layers of felt of moderate thickness, the thicker the layers the lower being the frequency at which they become effective absorbers. Preferably, the felt or other similar porous lining is made of grad- I ually :varying thickness with the thickest part thereof at the edge, or mouth, of the reflector and the lining extending inwardly of the reflector a distance such as to leave a relatively small area at the center of the reflector free from covering.

'As a result, the outer, or lined, portion of the reflector will not reflect much high frequency sound, and the directivity at the high frequencies can be reduced to the desired point. On the be reflected to the, microphone in spite of the felt lining, will utilize larger areas of the reflector,

- and thereby sufficient overall directivity and reinforcement will be obtained. In some cases, it may be found desirable to alter the curvature of the reflector, departing slightly from a true parabola, to compensate for the shift of phase of the waves that are reflectedfrom the felt covered area.

According to another form of-my present invention, I employ a suitable sound lens instead of a reflector and place the microphone at substantially the focus of the lens. The outer portions of the lens (that is, in a region adjacent the periphery thereof) can be covered'with an annulus of felt or other suitable sound absorbing material other hand, the low frequency sounds, which will provide an improved sound collecting system of gradually varying thickness to provide selective s5 larity in the appended claims. The inventionitself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will be best understood from the following description of several embodiments thereof, when read in connection with the accompanying drawing in which Figure 1 is a view showing a sound collecting system which includes a reflector and microphone according to conventional practice,

Figure 2 is a similar view of a sound collecting system according to my present invention,

Figure 3 is a view similar to Fig. -1 but showing diagrammatically a sound collecting lens instead of a reflector,

Figure 4 is a view similar to Fig. 3 but arranged according to my present invention, and

Figure 5 is a view similar to Fig. 4 but showing a somewhat different embodiment of the modification of my invention shown in Fig. 4.

Referring more particularly to the drawing, wherein similar reference numerals designate corresponding parts throughout, there is shown, in Fig. 1, a parabolic reflector I at substantially the focal point 3 of which is located a suitable microphone or other sound wave responsive device 5. The reflector I is made of large diameter at its base or mouth 1, in order, as pointed out previously, to obtain directivity for low frequency sounds. Such a system is representative of prior artsystems and is open to the objections previously mentioned, particularly that the reflector I is so sharply directive for high frequency sounds that it cannot be aimed accurately enough toward the sound source which is being picked up.

To remedy this defect, I have provided the system shown in Fig. 2. The parabolic reflector I of this system may be similar to that shown in Fig. 1, but it has a diameter at the mouth 1 of the order of at least one-wave length of the lowest frequency to be concentrated, and it is provided on its reflecting surface with a covering or linings of felt or other suitable porous.

material. ,The lining 9 is of gradually varying thickness and extends substantially from the mouth 1 of the reflector I inwardly a distance short of the central region II of the reflector I, leaving the central region II entirely uncovered. The, lining 9 is thickest over the zones adjacent the mouth 1 and thinnest adjacent the uncovered central region II of the reflector. It is the property of layers of felt of moderate thickness that they absorb high frequency sounds much more readily than the low frequency sounds, and the thicker the layer the lower is the frequency at which it becomes an effective absorber. By covering the reflecting surface of the reflector 'I with a lining 9 of gradually varying thickness, with the thickest portion thereof at the edge, or mouth, of the reflector, and with the central area II left uncovered, the covered portion of the reflector will not reflect much high frequency sound. Accordingly, the

directivity of the system at the higher frequencies is reduced to the desired point, it being apparent that there is less reflection of the high frequency. waves toward the microphone 5 adjacent the base or mouth I than adjacent the uncovered region II, and by far-most of the reflection of the high frequency waves toward the microphone 5 taking place at the uncovered region II. Since, however, the low frequency waves are reflected in spite of the felt lining 9,. it will be apparent that much larger areas of the reflector I will be utilized in reflecting the low frequency sounds toward the microphone, and thus sufficient over-all directivity will be obtained without having any undesirable concentration and excessive directivityin the high frequency range. The effect, it will be seen, is equivalent to providing a large diameter mirror for low frequency sounds andv progressively smaller mirrors for higher frequencies.

In Fig. 3, there is shown a system functionally similar to that of Fig. 1, but employing a sound lens 2| instead of the reflector I, the microphone 5 being located substantially at the focal point 23 of the sound lens. The sound lens 2| is constituted by a pair of very thin membranous elements 2.5 and 21, such as sheet rubber or oiled silk, sealed at their peripheries to provide a hollow structure'or chamber therebetween filled with a heavy gas. The system shown in Fig. 3 is objectionable similarly to that shown in Fig. 1. In accordance with my invention, I provide an annular covering 29 of felt or the like, on either one or both of the membranes 25, 21, as may be desired, only the member 25 in Fig. i being shown as provided with the felt covering 29. As in the system of Fig. 2, the felt covering 29 extends from .substantially the periphery of the lens 2| radially inwardly a distance short of the radius of the lens, so as to provide a central region 3I which is uncovered. The annular covering 29, like the lining 9, is of gradually varying thickness, being thickest adjacent the periphery of the lens and thinnest adjacent the central, uncovered region 3 I, and providing selective absorption of the high frequency sounds, the absorption being greatest adjacent the periphery of the lens.

In Fig. 5, I have shown a modified form of the last-described embodiment of my invention. In this modification, the felt covering 29 is dispensed with and the membranes 25 and 21 are made of gradually varying thickness from the periphery to the central regions 3| which is" general preferable in a lens such as shown in- Fig. 5 to apply the thickening for the most part to the membrane on the side toward the source of sound, thus preventing the undesired sound from entering the lens, where it would be subjected to successive internal reflections, and perhaps in some measure reach the microphone. With the embodiments of my invention shown in Fig. 4 and Fig. 5, the high frequency waves are reflected more and more as the periphery of the lens is approached, so that less of the high frequency sounds will be transmitted through the lens. On.- the other hand, the low frequency waves are affected much less by the thickened portions of the walls 25 and 21 or by the covering 29, as the case may be, and practically all of the low frequency waves are transmitted by the lens 2|. Again, therefore, animprovedover-all directivity will result.

Although I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many other modifications are possible. I therefore desire that my invention shall not .be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A sound collecting system comprising means having a predetermined focal point for receiving and directing sound waves from said direction to a predetermined focal point, said means having a central region and a periphery, and sound wave responsive means located at substantially said focal point and arranged to be actuated by said sound waves, said first-named means including a variable frequency discriminating means for directing waves of certain frequencies to said focal point more readily than waves of certain other frequencies, and said frequency discriminating means extending from substantially-said periphery to said central region.

2. A sound collecting system comprising means for receiving and directing sound waves from a predetermined direction to a predetermined focal point, said means at substantially said focal point and arranged to be actuated by said sound ried by said first-named means adapted to direct sound waves of different frequencies to said focal point as a function of its thickness, said last-named means extending from said'periphery to a region short of said central'region and having a gradually varying thickness with its thickest portion adjacent said periphery and its thinnest portion closest to said central region. 3. A sound collecting system comprising a substantially parabolic reflector, a microphone 10- including a central region and a periphery, sound wave responsive means located waves, and means car- I cated at substantially the focal point thereof, and a lining of felt on the reflecting surface of said. reflector, said lining extending from sub stantially the mouth of said reflector to a region short of the central region thereof, and said lining being of gradually varying thickness with its thickest portion adjacent said mouth and its thinnest portion adjacent said central region.

4. A sound collecting system comprising a sound lens, a microphone located at substantially the focal point thereof, a covering of felt on said sound lens, said covering'extending from substantially the periphery thereofradially'inwardly a distance short of the radius of said lens whereby to provide an uncovered central region on said lens, and said covering being of gradually varying thickness with its thickest portion adjacent said periphery and its thinnest portion adjacent said central region.

5. A sound collecting system comprising,. in combination, a sound lens and a microphone located at substantially the focal point thereof, said sound lens comprising a sealed, hollow membranous element having a; gas therein, and said membranous element being of varying thickness.

6. A sound collecting system comprising, in combination, a sound lens and a microphone located at substantially the focal point thereof, said sound lens comprising a sealed, hollow membranous element ha g a gas therein, and said element being of gra ually varying thickness with its thickest portion adjacent the periphery, the

thickness of said element being gradually reduced radially inwardly a distance short of the radius of said lens and the central region of said member being of substantially uniform thickness and constituting the thinnest portion of saidelement.

EDWARD W. KELLOGG.

CERTIFICATE OF CORRECTION Patentlio; 2,216,919.;

October a, 19m.

EDWARD w. KELLOGG.

It is hereby certified that. error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 5, first column, line ll,'claim 1, for the word "said" read .--a predeterminedline 12, for "a predetermined" read -said--.; and that the said. Letters Patent should be read with this correction therein that the some mayconform to the record of the casein the Patent'Office.

si ned and sealed ems L en day'of'February, A.

(Seal) D. 19in.

Henry Van Arsdale, Acting Commissioner of Patents CERTIFICATE OF CORRECTION PatentvNo; 2,216,9h9. October 3, 191m.

EDWARD w. -KELLOGG.

It is hereby certified that. error appears in the printed specification of the above lnumb ered patent requiring correction as follows: Page 5, first column, line 11,'c1aim 1, for the word "said" read --a predeterminedline 12 for "a predetermix led" read --said-; and that the said. Letters Potent should be read with this correction therein theta the sanie may..conform to the record of the case in the Patent-Office.

Signed and se a1ed this J m day -of'1 ebr'u.:ir"y-, A. D. 19!;1.

fienry Van Arsdale, I Acting Commissioner of Patents. 

